A " repeat order" is one that is made again and again, i.e., repeatedly.
A " standing order" is one that is " permanent, rather than formed or created when necessary"
i.e., it is renewed automatically wheneever some condition is met, e.g., at the end of every month or quarter,when the inventory falls below a certain level, when the price reaches a certain point, etc. If your customer has a standing order for 10,000 units of your product every month, that means that order is renewed automatically each month, and the customer does not tell you s/he wants to reorder.
We also use " standing " in this sense in such combinations as"
-" a standing committee" = a permanent committee
-" a standing invitation " = you're always welcome to come (not just when I invite you).